1. Field of the Invention
The present invention generally relates to photolithography and associated methods and apparatus for exposing semiconductor substrates.
2. Description of the Related Art
Lithographic exposure apparatuses can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of photo-activated resist (i.e., photoresist) material. In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time.
The term “patterning device” as will be employed herein should be broadly interpreted to refer to a device that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate. The term “light valve” may also be used in this context. Generally, the pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). Examples of such patterning devices include:                (a) a mask: the concept of a mask or reticle is well known in lithography, and it includes reticle types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid reticle types. Placement of such a reticle in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the reticle, according to the pattern on the reticle. In the case of a reticle, the support structure will generally be a reticle table, which ensures that the reticle can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired;        (b) a programmable mirror array: an example of such a device is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that (for example) addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind; in this manner, the beam becomes patterned according to the addressing pattern of the matrix-addressable surface. The required matrix addressing can be performed using suitable electronic means. More information on such mirror arrays can be gleaned, for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193, which are incorporated herein by reference. In the case of a programmable mirror array, the support structure may be embodied as a frame or table, for example, which may be fixed or movable as required; and        (c) a programmable LCD array: an example of such a construction is given in U.S. Pat. No. 5,229,872, which is incorporated herein by reference. As above, the support structure in this case may be embodied as a frame or table, for example, which may be fixed or movable as required.        
For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a reticle and reticle table; however, the general principles discussed in such instances should be seen in the broader context of the patterning devices as set forth above. Also, the projection system may hereinafter be referred to as the “lens”; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a “lens”.
In current apparatuses, employing patterning by a reticle on a reticle table, a distinction can be made between two different types of machine. In one type of lithographic exposure apparatus, each target portion is irradiated by exposing the entire reticle pattern onto the target portion in one go; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatus—commonly referred to as a step-and-scan apparatus—each target portion is irradiated by progressively scanning the reticle pattern under the projection beam in a given reference direction (the “scanning” direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction. Because, in general, the projection system will have a magnification factor M (generally <1), the speed V at which the substrate table is scanned will be a factor M times that at which the reticle table is scanned. More information with regard to lithographic devices as here described can be gleaned, for example, from U.S. Pat. No. 6,046,792, incorporated herein by reference.
It is to be noted that the lithographic apparatus may also be of a type having two or more substrate tables (and/or two or more reticle tables). In such “multiple stage” devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Twin stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and WO 98/40791, incorporated herein by reference.
It will be appreciated that the wafer substrates Ws may be subjected to a variety of processes before lithographic apparatus exposes the reticle RE circuit pattern onto the wafer substrate W. For example, the wafer substrates Ws may be treated or coated with a layer of photo-activated resist (i.e. photoresist) material before exposure. Moreover, prior to exposure, the substrates Ws may also be subjected to cleaning, etching, ion implantation (e.g., doping), metallization, oxidation, chemo-mechanical polishing, priming, soft bake processes, and measurement processes.
The wafer substrates Ws may also be subjected to a host of post-exposure processes, such as, for example, post exposure bake (PEB), development, hard bake, etching, ion implantation (e.g., doping), metallization, oxidation, chemo-mechanical polishing, cleaning, and measurement processes. And, if several layers for each wafer substrate W is required, which is usually the case, the entire procedure, or variants thereof, will have to be repeated for each new layer.
The continual demand for smaller semiconductor devices, having smaller patterns and features on the wafer substrate, is pushing the limits on the optical resolution that can be achieved by lithographic exposure apparatus. Generally, the smallest size of repeatable feature (e.g., “half-pitch”) of a pattern exposed on wafer substrate W that can be optically resolved by lithographic exposure apparatus, depends on attributes of the projection lens PL and projection beam PB. In particular, the optical resolution for half-pitch feature size may be derived by using the simplified form of the Rayleigh resolution equation:k1=p0.5·NA/λ≧0.25  (1)
where:                p0.5 represents the repeatable feature size (e.g., “half-pitch”) in nm;        NA represents the numerical aperture of projection lens PL;        λ represents the wavelength of projection beam PB; and        k1 represents the optical resolution limit for half-pitch feature size.        
As indicated above, the theoretical optical resolution half-pitch lower limit k1 for 2-beam imaging, is 0.25. In an attempt to circumvent the k1=0.25 barrier, considerable efforts have been directed to develop expensive technologies that are capable of employing shorter wavelengths and/or higher numerical apertures, thus allowing production of smaller features while not violating the k1≧0.25 constraint.